Move the DNS resolver out of the NewPinger() function in order to allow
adjusting of IPv4 vs IPv6 DNS resolution before running. This also
allows the user to verify resolution.
* Create new `New()` method that returns a bare default struct.
* Create new `Resolve()` method.
* Call `Resolve()` from `SetAddr()`.
* Call `Resolve()` automatically from `Run()`.
* Remove unecessary private `run()` method.
Update ping command for simplifed return values of `NewPinger()`.
Signed-off-by: Ben Kochie <superq@gmail.com>
